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Preprints
https://doi.org/10.5194/acp-2019-1219
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1219
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Jan 2020

Submitted as: research article | 16 Jan 2020

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This preprint is currently under review for the journal ACP.

Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP

David S. Stevenson1, Alcide Zhao1, Vaishali Naik2, Fiona M. O'Connor3, Simone Tilmes4, Guang Zeng5, Lee T. Murray6, William J. Collins7, Paul Griffiths8,9, Sungbo Shim10, Larry W. Horowitz2, Lori Sentman2, and Louisa Emmons4 David S. Stevenson et al.
  • 1School of GeoSciences, The University of Edinburgh, EH9 3FF, UK
  • 2Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration (NOAA), Princeton, NJ08540, USA
  • 3Met Office Hadley Centre, Exeter, UK
  • 4Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
  • 5National Institute of Water and Atmospheric Research, Wellington, New Zealand
  • 6Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY USA
  • 7Department of Meteorology, University of Reading, UK
  • 8National Centre for Atmospheric Science, University of Cambridge, UK
  • 9Department of Chemistry, University of Cambridge, UK
  • 10National Institute of Meteorological Sciences, Seogwipo-si, Jeju-do, Korea

Abstract. We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from CMIP6/AerChemMIP simulations. Global OH changed little from 1850 up to around 1980, then increased by around 10 %, with an associated reduction in methane lifetime. The model-derived OH trend since 1980 differs from trends found in several studies that infer OH from inversions of methyl chloroform measurements; however, these inversions are poorly constrained and contain large uncertainties that do not rule out the possibility of recent positive OH trends. The recent increases in OH that we find are consistent with one previous study that assimilated global satellite-derived carbon monoxide (CO) over the period 2002–2013. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic Near-Term Climate Forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime, by about 15–40 %, which has been assumed to not change in most previous studies. We find that whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. All these factors, together with changes and variations of climate and climate-driven natural emissions, need to be included in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.

David S. Stevenson et al.

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David S. Stevenson et al.

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Latest update: 04 Jul 2020
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Short summary
We present historical trends in atmospheric oxidising capacity (OC) since 1850 from the latest generation of global climate models, and compare these with estimates from measurements. OC controls levels of many key reactive gases, including methane (CH4). We find small model trends up to 1980, then increases of about 10 % up to 2014, disagreeing with (uncertain) measurement-based trends. Major drivers of OC trends are emissions of CH4, NOx and CO; these will be important for future CH4 trends.
We present historical trends in atmospheric oxidising capacity (OC) since 1850 from the latest...
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